A fuel cell is an energy conversion device that converts the chemical energy of a fuel directly into the electrical energy, which has been studied and developed as the next generation energy source characterized by high energy efficiency and pro-environmental properties lowering the discharge of pollutants.
Among fuel cells, the polymer electrolyte membrane fuel cell (PEMFC) comes into the spotlight as a portable power supply, power supplies for automobile and home owing to its low operating temperature, fast drive and water-tightness which has been a target of the use of solid electrolyte. This fuel cell is a high power fuel cell having high current density, compared with those of other fuel cells, and has numbers of advantages such as; it is operated at under 100° C., it has simple structure, fast start, fast response and excellent durability, and it can use methanol or natural gas, in addition to hydrogen, as a fuel. Besides, owing to the possibility of miniaturization attributed to high output density, studies have been undergoing to develop the fuel cell as a portable fuel cell.
The polymer electrolyte membrane fuel cell is composed of two electrodes and a polymer membrane acting as an electrolyte. The polymer electrolyte membrane is activated by using polymer membrane, where sulfonic acid group (—SO3H) is introduced, as electrolyte and using hydrogen or methanol as a fuel. In particular, the polymer electrolyte membrane fuel cell using methanol as a fuel is called ‘direct methanol fuel cell (DMFC)’, which is the system converting methanol directly to electricity by electrochemical reaction, satisfying our expectation for simplification and loading response of the system.
In general, the electrolyte membrane used for the polymer electrolyte membrane fuel cell is divided into two groups; perfluorinated polymer electrolytes and hydrocarbon polymer electrolytes. The perfluorinated polymer electrolyte is chemically stable owing to the strong binding force between carbon and fluoride (C—F) and shielding effect which is the typical character of fluoride atom, and has excellent mechanical properties in addition to the excellent conductivity as a proton exchange membrane. Thus, the perfluorinated polymer electrolyte has been industrially used as a polymer membrane for the polymer electrolyte membrane fuel cell. Nafion (perfluorinated sulfonic acid polymer) provided by Du Pont, U.S.A. is one of representative examples for commercial proton exchange membrane, which has been most widely used today owing to its' excellent ionic conductivity, chemical stability and ion selectivity. Although the perfluorinated polymer electrolyte membrane has excellent capacity, it has problems of limitation in commercial use because of its high price, high methanol crossover and descending efficiency of the polymer membrane over 80° C. Thus, studies on hydrocarbon ion exchange membrane which can compete with the perfluorinated polymer electrolyte membrane in the aspect of price have been actively undergoing.
The polymer electrolyte membrane for fuel cell should be stable under any circumstances required for the operation of the fuel cell, so the available polymer is limited in aromatic polyether, etc. During the operation of fuel cell, electrochemical stress such as hydrolysis, oxidation, reduction, etc, causes decomposition of the polymer membrane, resulting in the decrease of the capacity of the fuel cell. So, an attempt has been made to apply polyetherketone or polyethersulfone poly arylene ether polymer having excellent chemical stability and mechanical properties to fuel cell.
U.S. Pat. No. 4,625,000 describes the post-sulfonation process of polyethersulfone as a polymer electrolyte membrane. However, the post-sulfonation of polymer has limitations in regulation of the distribution, the location and the number of sulfonic acid group (—SO3H) of the polymer backbone and has a problem of descending properties of the electrolyte membrane with the increase of sulfonic acid groups causing the increase of the water content in the membrane.
U.S. Pat. No. 6,090,895 describes the cross-linking process of a sulfonated polymer such as sulfonated polyetherketone, sulfonated polyethersulfone and sulfonated polystyrene, etc. However, it could not propose an effective way to produce thin film using the sulfonic acid polymer cross-linked as the above.
EP No. 1,113,17 A2 describes a block copolymer electrolyte membrane composed of blocks having and not having sulfonic acid. A block copolymer composed of aliphatic block and aromatic block was sulfonated by using sulfuric acid, which could not control the location and the number of sulfonic acid groups in the polymer backbone and caused break-down of aliphatic polymer bond during sulfonation.
Japanese professor Watanabe described in his paper (Macromolecules 2003, 36, 9691-9693) the method of introducing sulphonic acid group selectively into the location of fluorene in a polymer containing a fluorene compound by using chlorosulfonic acid (ClSO3H). However, the method has a problem of the decrease of physical properties, precisely, the degree of sulfonation is increased by the method, resulting in the increase of the content of water in thin film, reducing the physical properties of the polymer electrolyte.
US Patent No. 2004-186262 describes the preparation method for a multi block copolymer electrolyte membrane in which hydrophobic block composed of hydrocarbon and hydrophilic block composed of hydrocarbon and having ionic conductivity are cross-linked. According to the method, a copolymer in the form of —SO3K was converted into —SO2Cl by using thionylchloride (SOCl2), taking advantage of low solubility of a multi block copolymer, to produce thin film. The produced thin film was hydrolyzed again into a polymer thin film in the form of —SO3H to endow proton conductivity to the thin film. But, the method has also problems, even though it enables the production of polymer thin film having ionic conductivity from multi block copolymer, the production process is complicated, thionylchloride is a toxic material, and the mechanical integrity of the polymer thin film is far behind the requirement for the operation of fuel cell.